1. Field of the Invention
The present invention relates to a noise reducer for effectively reducing noise included in a video signal and, more particularly, to a noise reducer capable of improving the S/N of a received image of the MUSE system.
2. Description of Related Art
In order to achieve transmission of a high definition television signal (HDTV) through a transmission medium of a narrow band, there is developed the transmission system MUSE (Multiple Sub-Nyquist Sampling Encoding) in which the signal to be transmitted is sub-sampled so as to be compressed in its signal bandwidth. In such system the still video image region and the moving video image region are subjected to different manners of signal processing so that deterioration in the resolution of the video image on a display picture area becomes less noticeable visually.
As a noise reducer for use in a receiving system of such MUSE system for suppressing mixed noise in a video signal while being transmitted through a signal transmission line, there is known, for example, a circuit as shown in FIG. 4.
In the MUSE decoder of such receiving system, there are provided a signal processing block 10 for suppressing noise in a received input signal S and two delay circuits 20A and 20B each for introducing one-frame delay.
The above described signal processing block 10 is provided with a subtractor 11 for detecting a difference signal between a signal delayed by a two-frame period and an input signal, hereinafter referred to as "two-frame difference signal", a coefficient multiplier 12 for providing the output signal of the subtractor 11 with a predetermined characteristic, and an adder 13 for deducting the output of the coefficient multiplier 12 from the original video signal thereby achieving noise reduction.
In the MUSE system, one picture area of still video image is obtained through transmission of four fields and, hence, the two-frame difference signal can be considered as a noise signal component.
Since the output of the subtractor 11 thus includes the noise component, a noise reducer can be established by having this signal supplied, through the coefficient multiplier 12, to the adder 13 so that the noise component is deducted from the original signal.
The coefficient multiplier 12 is arranged such that its nonlinear characteristic is changed by a noise level control signal C obtained by detecting the level variation component of a reference signal superimposed, for example, on the clamp level of 563H or 1125H of the received video signal, whereby the noise quantity to be canceled is controlled to increase when the noise quantity of the transmitting system is increased.
Further, the coefficient multiplier 12 is provided with a characteristic to suppress its output level when the two-frame difference signal is greater than a predetermined value, taking such difference signal for a moving video image region.
The above described noise reducer can vary its noise suppression according to whether the transmitting system is good or bad, but when the S/N of the transmitting system is good, for example, its suppression becomes lower. Hence, there has been a difficulty that it cannot exhibit a sufficient effect as a noise reducer against the noise originally included in the video signal. For example, it cannot suppress the noise included in a low quality video signal prior to transmission.
Further, a slightly moving portion is taken as noise and the noise reducer operates on such portion. Hence, there has been a difficulty that a deterioration in the quality of image, corresponding to a double image in an moving video image, is produced.